Interferometric Measurements of Temperature-Dependent Optical Absorption in Low-Loss SiN Membranes for Laser Sails

We have been evaluating candidate low-optical-loss materials for laser-light sails, which have been proposed as a mode of interstellar travel, and must reflect driving laser light while being highly robust to laser damage. For proposed laser sail missions, absorption coefficients of approximately 10^-2 cm^-1 or lower are required to maintain sail integrity at the highest laser intensities¹. One candidate laser sail material is silicon nitride (SiN) suspended membranes^1,2, which can be formed in different stoichiometries, for example stoichiometric Si₃N₄ and silicon-rich SiN_x (x~1). The low levels of optical loss in such membranes cannot be measured using conventional measurements such as ellipsometry.

In this work, we use self-referencing photothermal common-path interferometry (PCI) to study absorption in thin, free-standing SiN membranes. PCI is a continuous-wave pump-probe technique for measuring optical absorption in low-loss materials³, wherein we interferometrically measure the thermo-optic effect caused by a chopped high-power pump laser (1 W) using a less powerful probe laser (2 mW), with the probe's detector synchronized to the chopper via a lock-in amplifier. We show a new self-referencing technique for PCI by coating the sample with monolayer graphene that has an easily measurable absorbance of the order of 1%, slightly different from the well-known value of 2.3% due to Fabry-Perot effects in the membrane. Using this reference, we demonstrate a self-referencing PCI technique to find absorptivity values in Si₃N₄ and SiN_x (x~1). We found the room-temperature absorption coefficient of Si₃N₄ at 1064 nm to be (2.09 ± 0.76) × 10^-2 cm^-1, and that of SiN_x (x~1) to be 7.94 ± 0.50 cm^-1. These results not only point to the suitability of Si₃N₄ as a candidate material for laser sails, but also show self-referencing PCI as a viable method to measure loss in low-loss, free-standing membranes.

In addition, many dielectrics are known to suffer from increased optical absorption at higher temperatures due to bandgap narrowing, leading to thermal runaway in laser sail models when compounded with two-photon absorption¹. We will present high-temperature absorptivity measurements of Si₃N₄ membranes – using small resistive heaters with feature sizes down to 70 μm – fabricated on top of the membranes using metal evaporation through a shadow mask to achieve localized heating on the membranes only, which avoids introduction of noise to PCI measurements caused by the heating of its optical components.

References
¹ G.R. Holdman, G.R. Jaffe, D. Feng, M.S. Jang, M.A. Kats, and V.W. Brar, Adv Opt Mater 10(19), 2102835 (2022).
² H.A. Atwater, A.R. Davoyan, O. Ilic, D. Jariwala, M.C. Sherrott, C.M. Went, W.S. Whitney, and J. Wong, Nature Materials 2018 17:10 17(10), 861–867 (2018).
³ A. Alexandrovski, M. Fejer, A. Markosian, and R. Route, in Solid State Lasers XVIII: Technology and Devices, (SPIE, 2009), p. 71930D.